Imaging deviation compensation

ABSTRACT

Imaging offset problems in imaging systems, such as electrophotographic (EPG) printers and copiers, are overcome. Imaging offset results from misaligned exposure units that, when uncompensated, produce dots on a photoreceptor belt at exposure positions that are offset from ideal dot positions. An imaging-offset compensating method of the invention first determines the imaging offset, which is a distance that may include a magnitude and a direction. The imaging offset is determined with respect to the ideal dot position. A time factor is then determined based on the magnitude of the imaging offset for each exposure unit. The time at which each exposure unit is actuated is modified by a respective time factor so that a dot produced by each exposure unit matches the ideal dot location thereof.

REFERENCE TO OTHER APPLICATIONS

[0001] This is a division of U.S. nonprovisional utility patentapplication Ser. No. 09/718,069, filed Nov. 21, 2000, filed byWen-hsiung Lee, assigned to the present applicant.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to imaging systems and,more particularly, to a method and a system for compensating for anoffset of a dot or series of dots in an imaging system, such as in anelectrophotographic system (e.g., color printers).

[0004] 2. Description of the Related Art

[0005] One of the difficulties in designing imaging systems such aselectrophotographic (EPG) printers, inkjet printers, and laser printersis overcoming a problem known as “imaging offset.” This problem resultsin poor image quality in both monocolor (i.e., black and white) andmulticolor systems. To better understand the background of this problem,imaging offset will be explained with reference to an EPG printer.

[0006] In general, imaging offset results from physically misalignedlight sources or exposure units, e.g., light-emitting diodes (LEDs),that are used in the EPG printer to form an image. Light sources maybecome misaligned from either errors during the manufacturing process ordamage to the EPG printer after manufacture. Additionally, to ensurethat light sources are perfectly aligned, strict manufacturingtolerances must be maintained, which is difficult and increases costs.

[0007] In an EPG system such as a printer or a copier, an electrostaticlatent image is formed on a charged surface of a photoreceptor byexposing the photoreceptor with a high-intensity light source such as anLED array. Prior to exposure, the surface of the photoreceptor isuniformly charged. The LEDs then create a charged pattern (known as a“latent image”) corresponding to the image that is to be printed. Thelatent image is then developed into a toner image by adhering chargedtoner particles to the charged pattern on the photoreceptor. The tonerimage is transferred to paper using an electrostatic transfer process.The toner image is then fused to the paper by heat. The photoreceptor isthen cleaned prior to the next imaging cycle of the system.

[0008] Imaging offset occurs in the EPG imaging process at the pointwhen the LEDs create the charged pattern. As mentioned above, LEDs maybe misaligned during the manufacturing process (e.g., mounting of LEDchips) or after the manufacturing process due to damage to the EPGprinter or any intermediary device (e.g., SFL error). Misaligned LEDsare offset from an ideal linearity by different distances. An imageresulting from this nonlinear array of LEDs is of poor quality.

[0009] Imaging offset similarly occurs in multicolor imaging. MulticolorEPG copying and printing requires the EPG process explained above formonocolor images to be repetitively performed for each color. Differentstations for each of the different colors (e.g., yellow, magenta, cyan,and black) apply toner of a specific color. In multicolor imaging, thetoner powder images should be superimposed upon each other in nearperfect registry (or alignment) to produce high-quality color images. Ifmisregistration occurs, the color images may blur, and color hue shiftsmay occur. Misaligned LEDs therefore cause these registration problems.

[0010] In view of the foregoing, a need exists for a method and a systemfor compensating for imaging offset to avoid the linearity andregistration problems described above.

SUMMARY OF THE INVENTION

[0011] The present invention overcomes the imaging-offset drawbacks ofconventional imaging systems and provides imaging systems that produceclear, crisp, and true-color images free from imaging offset.

[0012] According to one aspect of the present invention, a method ofcompensating for imaging offset of a dot produced by an exposure unit ona substrate in an imaging system. The dot has an uncompensated dotposition and an ideal dot position. The uncompensated dot position isout of alignment with the ideal dot position. To compensate for thismisaligned, the imaging offset is determined as a distance between theideal dot position and the uncompensated dot position. Based on thedetermined imaging offset, the uncompensated dot position is thenmatched to the ideal dot position.

[0013] One of the advantages of the present invention is that imagingoffset is substantially eliminated in the imaging process. Accordingly,images produced by, for example, printers and copiers are clear, crisp,and free of errors. In addition, images produced by color systems do notsuffer from registration problems of one color upon the other.Accordingly, color image provide clear, true colors.

[0014] Cost savings is another advantage of the present invention. Morespecifically, conventional approaches attempting to reduce imagingoffset by apply strict manufacturing tolerances during the productionof, for example, light-emitting diode (LED) printer heads (LPHs). Thisstrict adherence is expensively and, ultimately, falls short of successbecause of the size and number of diodes in the LPHs. In accordance withthe present invention, imaging offset may be corrected regardless of themisaligned in diodes of the LPHs. Accordingly, inexpensively producedLPHs may be used to produce images of the highest quality.

[0015] The matching of the dot to the ideal dot position may beaccomplished, for example, by delaying a formation of the dot on thesubstrate by an amount of time corresponding to the imaging offset.Alternatively, a time factor based on the imaging offset may bedetermined. The exposure unit may then be actuated to produce a dot at atime modified by the time factor. The time factor may be based on both amagnitude of the distance of the imaging offset, as well as a directionof the imaging offset.

[0016] Another advantage of the present invention related to theapplicability of its methodology. More specifically, an on-boardsoftware module may implement the compensation method. In alternativeembodiments, the method of the present invention may be performed from aremote location. In this embodiment, an imaging system is incommunication with a processor that causes the exposure unit to beactuated so that the dot is produced at the ideal dot position.

[0017] Other aspects, features, and advantages of the present inventionwill become apparent, as the invention becomes better understood byreading the following description in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A more complete appreciation of the invention and many of theadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the detailed description when considered inconnection with the accompanying drawings, wherein:

[0019]FIG. 1 is a diagram view of an exemplary embodiment of anelectrophotographic (EPG) system configured in accordance with theprinciples of the present invention;

[0020]FIG. 2A is a diagram view of an ideally aligned array of exposureunits;

[0021]FIG. 2B is a diagram view of ideally aligned dots exposed by theexposure units of FIG. 2A;

[0022]FIG. 3A is a diagram view of an array of exposure units that areout of alignment;

[0023]FIG. 3B is a diagram view of dots exposed by the exposure units ofFIG. 3A that suffer from imaging offset;

[0024]FIG. 3C is a diagram view of aligned dots that have beencompensated in accordance with the principles of the present invention;

[0025]FIG. 4A is a diagram view of an array of exposure units that areout of alignment with respect to an ideal alignment range;

[0026]FIG. 4B is a diagram view of dots exposed by the exposure units ofFIG. 4A that suffer from imaging offset;

[0027]FIG. 4C is a diagram view of aligned dots that have beencompensated in accordance with the principles of the present invention;

[0028]FIG. 5 is a schematic view of an exemplary embodiment of a delaydevice of the present invention;

[0029]FIG. 6 is a block diagram of a software module configured inaccordance with the principles of the present invention;

[0030]FIG. 7A is a block diagram of an exemplary software moduleconfigured such that a compensation function is performed after imagedata is stored in an image buffer, in accordance with principles of thepresent invention;

[0031]FIG. 7B is a block diagram of an exemplary software moduleconfigured such that a compensation function is performed before imagedata is stored in an image buffer, in accordance with principles of thepresent invention; and

[0032]FIG. 8 is a block diagram of an exemplary computer system forimplementing the compensation methodology of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Referring to the drawings in more detail, an exemplary imagingsystem 100 configured in accordance with the principles of the presentinvention is shown in FIG. 1. The imaging system 100 of the inventionincludes hardware and software for producing images, such as colorphotocopies, that are crisp, clear, and free from imaging offset.

[0034] For the purposes of this description, exemplary embodiments for amethod and a system of the present invention for compensating forimaging offset in an imaging system are described in detail withreference to an electrophotographic (EPG) system. It is understood,however, that the method and the system of the present invention iswidely applicable to imaging systems that incorporate a variety ofimaging technology, such as lasers, inkjet, tonerjet, bubblejet, andother techniques for creating an image.

[0035] Exemplary imaging system 100 includes an EPG subsystem 102enclosed within a housing 104. The EPG subsystem 102 and the housing 104may be combined and configured to form various embodiments of theimaging system 100, such as a printer (monocolor or multicolor), acopier (monocolor or multicolor), a scanner, or any image-forming systemthat is a combination of these devices. For the purposes of thisdescription, the imaging system 100 is depicted as a multicolor printerin FIG. 1.

[0036] Exemplary EPG subsystem 102 includes a flexible, transparentphotoreceptor belt 106 with an outer surface 108 and an inner surface110. The photoreceptor belt 106 is driven in a continuous path in adirection indicated by arrow A at a velocity v by rollers 112 a and 112b. In one embodiment, the photoreceptor belt 106 is a substrate uponwhich images are formed as described in detail below. Along a portion ofthe path are disposed a plurality of printing stations 114 a, 114 b, 114c, . . . , 114 n. For example, four printing stations 114 are shown inFIG. 1, although it is understood that the number of printer stationsmay vary in other embodiments of the invention, such as a single printerstation for monocolor EPG systems.

[0037] Each of the printing stations 114 respectively produces adifferently colored image, in superimposed relation, on the outersurface 108 of the photoreceptor belt 106. In alternative embodiments,for example, where one printing station is used, the colors (e.g.,yellow, magenta, cyan, and black) may be superimposed to producedifferently colored images by passing the photoreceptor belt 106 pastthe single printing station four times, rather than only once when fourprinting stations are used. One skilled in the art will understand thatvarious numbers of printing stations may be used to create color images,with only the number of passes of the photoreceptor belt 106 beingchanged.

[0038] Each of the printing stations 114 a-114 n includes a coronacharging unit 116 a, 116 b, 116 c, . . . , 116 n disposed adjacent to ornear the outer surface 108 of the photoreceptor belt 106. The chargingunits 116 apply a uniform charge to the belt 106. Located downstreamfrom each charging unit 116 is a light source 118 a, 118 b, 118 c, . . ., 118 n, for example, a laser or a light-emitting diode (LED) printerhead (LPH). The light sources 118 are disposed at or near the innersurface 110 of the photoreceptor belt 106. The light sources 118 includeexposure units such as LEDs that are selectively actuated to projectlight corresponding to a single-color image on the photoreceptor belt106, thereby discharging, at appropriate locations on the outer surface108, the uniform electrostatic charge provided by the charging units 116to produce an electrostatic charge image.

[0039] According to one particular embodiment of the invention,exemplary imaging system 100 includes an EPG module 120 that isconfigured to be releasable engageable with and, therefore, removablefrom the EPG subsystem 102. The removable EPG module 120 may include thephotoreceptor belt 106, the rollers 112, and the light sources 118.Exemplary module 120 may also include a handle (not shown) to facilitatethe removal of the EPG module 120 from the imaging system 100. Theremovably configured EPG module 120 allows easy maintenance of the EPGsubsystem 102 and replacement with another EPG module, if necessary ordesired. A replacement EPG module may be identical to the EPG module 120to be replaced or, in alternative embodiments, may include a differentnumber or type of light sources, a different type of photoreceptor belt,or different rollers. In addition, in view of rapidly advancingtechnology, the removable EPG module 120 allows an end user to upgradeto a later-developed module of improved quality, faster speed, higherresolution, and so on.

[0040] With additional reference to FIG. 2A, each light source 118 mayincludes a plurality of exposure units 122 a, 122 b, 122 c, . . . , 122m configured in a linear array. The exposure units 122 are selectivelyactuated to emit light, corresponding to an image to be formed, that isincident on the photoreceptor belt 106 (not shown in FIG. 2A) movingadjacent to the exposure units 122 as indicated by arrow A. The emittedlight exposes the photoreceptor belt 106 with the image to be formed bypassing through the belt 106 and discharging the uniform electrostaticcharge provided by the charging units 116 at appropriate locations onthe outer surface 108 of the belt 106. A charged pattern known as alatent image is formed on the outer surface 108 of the photoreceptorbelt 106.

[0041] The exposure units 122 of each light source 118 are ideallyaligned and linear in configuration within an alignment range R as shownin FIG. 2A. When actuated, the aligned exposure units 122 respectivelyproduce correspondingly aligned dots 124 a, 124 b, 124 c, . . . , 124mon the moving photoreceptor belt 106 as shown in FIG. 2B. Each dot 124is an electrically discharged area on the photoreceptor belt 106. Thecollective arrangement of dots 124 defines a latent image of the imageto be printed on a sheet of paper. FIG. 2B illustrates ideal dotpositions 124 ai, 124 bi, 124 ci, . . . , 124 mi, which are thepositions on a photoreceptor belt at which the dots 124 are perfectlyaligned and linear within an exposed alignment range R′ and, therefore,do not suffer from imaging offset.

[0042] In reality the exposure units 122 are not perfectly aligned asshown in FIG. 2A but are, rather, misaligned as shown in FIG. 3A. Asmentioned above, imaging offset results from a misaligned of theexposure units 122. Any number of the exposure units 122 of FIG. 3A areout of alignment and offset from the alignment range R by an alignmentoffset L. The misaligned exposure units 122 produce dots that arecorrespondingly offset from the ideal dot position and the exposurealignment range R′, as shown in FIG. 3B, if the exposure units 122 areactuated without compensation, which offset is known as imaging offset.For the purposes of this description, each of the exposure units 122 hasan exposure unit position that is defined as the physical location ofeach exposure unit within the array of exposure units.

[0043] For the purposes of this description, imaging offset O is thedistance between an ideal dot position shown in FIG. 2B and the positionexposed by a misaligned exposure unit of FIG. 3A, which offset anduncompensated dot position is shown in FIG. 3B and indicated byreference number 124 with an “o” subscript. For example, imaging offsetO(b) is the distance the uncompensated dot 124 bo is offset from theexposure alignment range R′ and, therefore, the corresponding ideal dotposition 124 bi. Each imaging offset O corresponds to and results froman alignment offset L of a respective uncompensated and misalignedexposure unit 122.

[0044] As shown in FIG. 3A, each alignment offset L has a magnitude anda direction. For the purposes of this description, a positive (+)direction and a negative (−) direction are defined as respectively shownby arrow P and arrow N in FIG. 3A. For example, alignment offset L(b) ofexposure unit 122 b has a magnitude of |L(b)| in the positive (+)direction, while alignment offset L(m−1) of exposure unit 122(m−1) has amagnitude of |L(m−1)| in the negative (−) direction. Thismagnitude-and-direction convention of the alignment offsets L isemployed analogously herein for the imaging offsets O.

[0045] In view of the forgoing, if the alignment offset L is notcompensated, then the misaligned exposure units 122 of FIG. 3A producethe uncompensated dots 124 o of FIG. 3B. According to the presentinvention, the respective alignment offset L of the misaligned exposureunits 122 are compensated for and, therefore, produce compensated dots124 c within the exposure alignment range R′ as shown in FIG. 3C, asdiscussed in detail below.

[0046] According to the present invention, imaging offset is correctedby first determining the ideal dot positions 124 ai, 124 bi, 124 ci, . .. , 124 mifor each exposure unit 122 a, 122 b, 122 c, . . . , 122 m,which may be done at any time during the manufacturing or thepost-manufacturing process. When determined, the ideal dot positions 124i may then be either stored in a storage device of the image system 100or communicated to the imaging system 100 from a remote location (e.g.,over a network connection). A subsequent step in correcting imagingoffset is determining the magnitude and the direction of the imagingoffsets O(a), O(b), O(c), . . . , O(m) between the ideal dot position124 i and the uncompensated dot position 124 o for each of the exposureunits 122.

[0047] The imaging offset O produced by each exposure unit 122 may bedetermined during the manufacture of the light source 118, the EPGsubsystem 102, or the entire image system 100. If carried out at themanufacturing stage, the imaging offsets O may be determined by firstlycomparing the respective positions of the exposure units 122 to theknown ideal alignment range R to determine the alignment offsets L. Theimaging offsets O may then be respectively determined from the alignmentoffsets L. The magnitude of each imaging offset O may either directly orproportionally correspond to the magnitude of a respective alignmentoffset L, as follows:

|O(x)|=k|L(x)|,

[0048] where k is a proportionality factor greater than zero and x is aninteger from 1 to m (as exemplified by the plurality of exposure units122 a-122 min FIGS. 2 and 3). The proportionality factor k may beconstant for the plurality of exposure units 122 or may have a uniquevalue for each respective exposure unit 122. The direction of eachimaging offset O directly corresponds to that of a respective alignmentoffset L.

[0049] After determining magnitude and direction, compensation forimaging offset for each dot is accomplished by matching theuncompensated dot position 124 o to the ideal dot position 124 i. Forthe ideally aligned array shown in FIG. 2A, each exposure unit 122 isactuated at an ideal actuation time to result in the ideal dot positions124 i of FIG. 2B. According to an exemplary embodiment, a time factor Dtis incorporated to the EPG subsystem 102 and, more specifically, intothe light sources 118 to modify the ideal actuation time of eachexposure unit 122 depending upon the imaging offset O thereof.

[0050] More specifically, as the photoreceptor belt 106 moves in thedirection indicated by arrow A at a know velocity v, and as themagnitude and the direction of the imaging offset O for each exposureunit 122 is known, then the time factor Dt for any exposure unit 122 xmay be determined by:

Dt(x)=O(x)÷v.

[0051] The magnitude of each imaging offset O determines the amount oftime t to modify the ideal actuation time for each exposure unit 122,while the direction of each imaging offset O determines whether theexposure unit 122 is actuated earlier or later than the ideal actuationtime thereof.

[0052] For example, if the imaging offset O for exposure unit 122 bhas amagnitude of 0.1 millimeter (mm) in the positive (+) direction, and ifthe velocity v of the photoreceptor belt 106 is 100 mm per second, thenthe time factor Dt of exposure unit 122 b is:

Dt(122 b)=(0.1 mm)÷(100 mm/s)=0.001 s=1 ms.

[0053] In addition, if the imaging offset O for exposure unit 122(m−1)has a magnitude of 0.08 mm in the negative (−) direction, then the timefactor Dt of exposure unit 122(m−1) is:

Dt[122(m−1)]=−(0.08 mm)÷(100 mm/s)=−0.0008 s=−0.8 ms.

[0054] Accordingly, as the photoreceptor belt 106 passes by the array ofexposure units 122, exposure unit 122(m−1) will actuate 0.8 ms soonerthan an ideal actuation time to compensate for imaging offset O(m−1),while exposure unit 122 b will actuate 1 ms later than an idealactuation time to compensate for imaging offset O(b). It follows thatthe time factor Dt for each exposure unit 122 has a sign (i.e., eitherpositive or negative) that is indicative of the direction of the imagingoffset O for each exposure unit 122.

[0055] This pre-actuation and post-actuation of exposure units 122 fromthe ideal actuation time results in respective compensated dots 124 ac,124 bc, 124 cc, . . . , 124 mc that are aligned within the exposurealignment range R′ as shown in FIG. 3C. The compensated dots 124 cresult in an image formed on sheet material by the imaging system 100with high resolution and clarity. Furthermore, in multicolor imagingsystems, each compensated dot is properly registered to result in truecolor. Compensating for the imaging offset O may be accomplished usingboth firmware or software as described more fully below.

[0056] Rather than being a quantity of time as described above, the timefactor may be calculated as a constant t that actuates an exposure unit122 to produce a dot within the exposure range R′. For example, the timeat which an exposure unit 122 is actuated for compensation, representedby t_(c), may be written as the product of a time constant t and anideal actuation time t_(i) as follows:

t _(c) =t′t _(i).

[0057] An alternative image-offset compensating embodiment of theinvention is described with reference to FIGS. 4A and 4B. Analogous tothat described above in relation to FIG. 3A, the exposure units 122 areout of alignment with respect to the alignment range R by an offset L.Accordingly, if actuated without compensation, the misaligned exposureunits 122 produce dots that are correspondingly offset from the idealdot position and the exposure alignment range R′, as shown in FIG. 4B.

[0058] According to this exemplary embodiment, rather than having thealignment range R fixed with each exposure unit having either a positive(+) offset or a negative (−) offset as described above, the alignmentrange R is adjusted or normalized to the position of a single one of theexposure units 122, for example, the exposure unit having an alignmentoffset L with the greatest magnitude in the positive direction. In theexample shown in FIG. 4A, either exposure unit 122 b or 122 m exemplifysuch an exposure unit. Accordingly, each alignment offset L has amagnitude. The direction of each alignment offset L is assumed to benegative. Correspondingly, the time factor Dt for each exposure unit isalways negative; that is, the time factor is always a time delay inactuating the exposure units 122 to compensate for the imaging offset O,thereby yielding compensated dots 124 c within the exposure range R′ asshown in FIG. 4C.

[0059] Rather than compensating for imaging offset O during themanufacturing stage as described above, compensation may take placeafter the exposure units 122 have been incorporated into the EPGsubsystem 102. According to this embodiment of the invention, imagingoffset is compensated during a single compensating stage for the EPGsubsystem 102, while the manufacturing is occurring. More specifically,after the exposure units have been manufactured and incorporated intothe light sources 118 (or into the EPG module 120 or the EPG subsystem102), the manufacturer performs the compensation method described aboveto compensate for the imaging offset during a single compensatingprocedure.

[0060] Another embodiment of the invention compensates for imagingoffset after the exposure units 122 have been incorporated into theimaging system 100, such as a multicolor printer. If imaging-offsetcompensation takes place after the manufacturing of the imaging system100, then a number of compensating procedures are available. Morespecifically, the compensating method of the present invention describedabove may be performed to compensate for imaging offset throughout thelife of the imaging system 100, the EPG subsystem 102, the light sources118, or the EPG module 120. Thus, if any post-manufacturing damageoccurs on any component of the imaging system 100 resulting inmisaligned exposure units, then the imaging system 100 may communicatewith software or firmware to perform the present compensating methodeither locally (i.e., within the EPG subsystem 102, the EPG module 120,or the light source 118) or remotely (i.e., over a data network, such asthe Internet, that is connected to the imaging system 100).

[0061] According to a further embodiment, imaging offset need not bedetermined physically within the imaging system 100. According to thisembodiment, the imaging system 100 is connected through communicationmedia (e.g., wire or wireless media) to a network, such as a local-areanetwork (LAN), a wide-area network (WAN), or the Internet. Althoughsoftware performing the present compensating methods may be storedwithin the imagine system 100, the EPG subsystem 102, the EPG module120, or the light sources 118, such software or firmware may be remotelylocated with the resulting imaging-offset compensation being transmittedto the imaging system 100 through the data network.

[0062] In alternative embodiments, the data network may be used tocommunicate specific identification information of the imaging system100, the EPG subsystem 102, the EPG module 120, or the light sources 118to a remote location, thereby receiving imaging-offset data specific forthat particular component. Accordingly, the imaging system 100 and anyof its components may have unique identification information such as aserial number or the like that specifically identifies the component,e.g., the EPG subsystem 102. After purchase, the imaging system 100 maybe connected to a network so that the identification information can betransmitted to a remote location with a computer system on which isstored the imaging-offset data for the particular EPG subsystem 102.Upon receipt of the unique identification information, the remotecomputer system may then transmit the imaging-offset data correspondingto the identification information to the EPG subsystem 102 to compensatefor the imaging offset. This embodiment is particularly useful when theEPG subsystem 102 or the EPG module 120 is replaced with another suchunit that would have different imaging-offset data corresponding to itsunique identification information.

[0063] The imaging-offset compensation methods of the present inventionmay be implemented using any device and methodology for determining theoffset, including distance and direction (if necessary), of eachexposure unit. In addition, any device or methodology for matching thedot position (using the time factor) to the ideal dot position may beemployed.

[0064] One exemplary hardware embodiment of an imaging-offsetcompensation unit configured in accordance with the principles of thepresent invention is shown in FIG. 5 and indicated by reference numeral130. Exemplary compensation unit 130 includes a plurality of flip-flops132, for example, four D flip-flops 132 a, 132 b, 132 c, and 132 d, anda 4′1 multiplexer 134. Generally, port DO of each flip-flop 132 isconnected to port DI of a subsequent flip-flop and to an input of themultiplexer 134. The CK inputs of the flip-flops 132 are coupled toLsync line 137, which is used to shift uncompensated image data (D_(n))into the flip-flops 132. “D_(n)” signifies that this uncompensated imagedata is to be placed on the nth position of an appropriate exposure unit122.

[0065] Exemplary compensating device 130 functions as a delaying devicethat is capable of delaying the formation of dots that collectively formthe latent image on the photoreceptor belt 106 by the time factorcorresponding to the imaging offset of each exposure unit 122. The timefactor descried above represents the unit of pixel line. For instance, atime factor of two represents a delay of two pixel lines. OFFSET lines 1n and 0 n, indicated in FIG. 5 by reference numeral 136, control thenumber of pixel lines to be delayed. In the exemplary embodiment shownin FIG. 5, up to three pixel lines of delay are provided. In otherexemplary embodiments, greater numbers of pixel lines of delay areprovided. For example, in one alternative embodiment, compensation unit130 includes three OFFSET lines, eight D flip-flops, and an 8-to-1multiplexer, to provide up to seven pixel lines of delay.

[0066] In operation, Lsync line 137 preferably shifts data Dn into therespective flip-flops 132 a, 132 b, 132 c, 132 d sequentially by pixelline. Data Dn is clocked into flip-flops 132 a, 132 b, 132 c, 132 d insequence, by a time factor corresponding to the imaging offset O of oneof the exposure units 122 defined by OFFSET signals input on lines 136.As mentioned above, the time factor corresponds to the amount of timeneeded to delay the formation of the dot by an exposure unit. Forexample, if a dot to be formed is desirably to be delayed by a factor of1, then line 136 sends a signal indicative of this factor to themultiplexer 134 which, in turn, directs the signal to line 138 and portI1. Flip-flop 132 b delays the signal on line 139 by a factor of 1 andthereafter returns the signal through line 140 to the multiplexer 134.The multiplexer 134 then outputs the delayed signal to port O and line142 that is connected to an input of an exposure unit. The delayedsignal actuates the exposure unit to form a compensated dot on thephotoreceptor belt 106. If a dot to be formed is desirably delayed by afactor of 2 or more, then the signal on line 136 selects the appropriateflip-flop 132 for the corresponding delay factor in order to properlydelay the formation of the dot. In other exemplary embodiments, othertypes of firmware are used to perform the function of matching the dotposition to the ideal dot position.

[0067] In other embodiments, software may be used in conjunction with aprocessor or as part of a computer system to determine the imagingoffset O and to match the dot position to the ideal dot position. Thesoftware may be stored on a storage device of any type, such as magneticmedia, optical media, DVDs, CD ROMs, RAMs, EPROM, EEPROM, or any othertype of media suitable for storing data or instructions. The softwaremay also act as a delaying device that is able, through computer code,to accept the offset and to delay the formation of the image by theoffset. It is noted that the hardware and software to implement themethod of the present invention may be located within the housing 104 oron various components of the imaging system 100, including on the EPGsubsystem 102, the light sources 118, or the EPG module 120.Alternatively, the compensation software or hardware may also be outsideof the housing 104 and in communication with the imagine system 100.

[0068] Returning to the description of the present invention shown inFIG. 1, exemplary imaging system 100 may include a supply tray 150 forholding sheet material 152 such as paper or transparencies. A roller 154engages one of the sheets 152 from the supply tray 150 and sends thesheet through a transfer station 156 where the latent image form on thephotoreceptor belt 106 is transferred to the sheet with toner. A fuser158 fixes the toner to the sheet and transfers the sheet with the fusedimage to an output tray 160.

[0069] For multicolor printing, the photoreceptor belt 106 is drivenpast the four printing stations 114 which produce four images of uniquecolor in superimposed relation on the outer surface 108 of the belt 106,which images collectively form a latent image. The latent image istransferred from the belt 106 to sheet material at the transfer station156. A cleaning unit 162 removes any residual developer and toner fromthe outer surface 108 of the photoreceptor belt 106 prior to passing bya first of the charging units (i.e., unit 116 a).

[0070] As mentioned above, a compensating module including softwareand/or hardware for determining the imaging offset O and thecorresponding compensation factor may be located at any appropriatelocation within the imaging system 100. For example, the compensatingmodule, which is indicated by reference numeral 164, may incorporatedwithin either the EPG module 120 as indicated at 164 a, one or more ofthe light sources 118 as indicated at 164 b, the EPG subsystem 102 asindicated at 164 c, or the housing 104 as indicated at 164 d. Asdescribed above, compensating for imaging offset results in an imagethat is clear and crisp with properly aligned registry, which isparticularly beneficial for multicolor images.

[0071] An exemplary embodiment of the compensating module 164 isillustrated in FIG. 6 as a software module. Exemplary softwarecompensating module 164 includes code for implementing the functionalitydescribed on each block of FIG. 6. For example, an offset module 170includes code and data regarding magnitude and other parameters fordetermining the imaging offset O in accordance with the methodologydescribed above. A match module 172 includes code for matching anuncompensated dot position to a respective ideal dot position tocompensate for the imaging offset. A communication module 174 includescode for communicating with the imaging system 100. The software coderepresented by the blocks may be stored on any storage device asdescribed above and may be run using any processor or computer system.

[0072] Exemplary software module 164 may be configured as a plurality ofcomputer-readable instructions stored on a computer-readable medium asknown in the art. Alternatively, the computer-readable instructions maybe located in an electronic signal that is transmitted over a datanetwork to perform the methods of the invention when loaded into acomputer system. The electronic signal may be transmitted via a datanetwork or via cable, satellite, cellular, or other suitabletransmitting means.

[0073] Exemplary compensation modules constructed according to thepresent invention have various configurations, particularly with respectto an image buffer in which image data is stored for processing. Forexample, FIG. 7A shows a configuration 175 in which image data, such asrendered image process (“RIP”) data, is stored in an image buffer 176before a compensation module 177 processes the data. In another example,FIG. 7B shows a configuration 179 in which the image data is processedby compensation module 177 before the data is stored in image buffer176. In both FIGS. 7A and 7B, the image data is compensated bycompensation module 177 using software and/or hardware configurations asdescribed above. For purposes of illustration, offset module 170 isshown as separated from compensation module 177, as opposed to theconfiguration of FIG. 6.

[0074] In FIG. 7A, the image data is stored in image buffer 176 beforebeing passed to compensation module 177. Image buffer 176 may beembodied, in one example, in conventional PC memory. The image data maybe transferred to and from image buffer 176 using direct memory access(“DMA”). After storage in image buffer 176, the image data is retrievedby compensation module 177 and compensated using techniques describedabove. Compensation module 177 outputs compensated image data to a lightsource 178, such as a LPH, to produce an electrostatic charge image. Theconfiguration in FIG. 7B functions similar to that of FIG. 7A, exceptthat the compensation module 177 is situated on the opposite side ofimage buffer 176 so that the image data is compensated before beingstored in image buffer 176. The configurations of FIGS. 7A and 7B areadvantageous because, in both cases, the compensation module 177 isseparated from the light source 178. By doing so, the cost of the lightsource is reduced significantly.

[0075] A computer system 180 configured in accordance with theprinciples of the present invention is illustrated in FIG. 8 with ahigh-level block diagram. Exemplary system 180 includes a processor 182and memory 184. Processor 182 may include a single microprocessor or aplurality of microprocessors for configuring the computer system 180 asa multi-processor system. Memory 184 may store instructions and data forexecution by processor 182. Depending upon the extent of softwareimplementation in the system 180, memory 184 may store executable codewhen in operation. Memory 184 may include, for example, banks of dynamicrandom access memory (DRAM) as well as high-speed cache memory.

[0076] Exemplary system 180 may also incorporate any combination ofadditional devices, including but not limited to a mass storage device186, one or more peripheral devices 188, an audio device 190, one ormore input devices 192, one or more portable storage medium drives 194,a graphics subsystem 196, a display 198, and one or more output devices200. For purposes of simplicity, the components shown in FIG. 8 areconnected via a single bus 202; however, the components may be connectedthrough one or more communication media as known in the art. Forexample, processor 182 and memory 184 may be connected via a localmicroprocessor bus; and the mass storage device 186, the peripheraldevices 188, the portable storage medium drives 194, and the graphicssubsystem 196 may be connected via one or more input/output (I/O) buses.As shown in FIG. 8, the light sources 118 are in communication with thecomputer system 180 for actuation of the exposure units 122 based on thetime factor.

[0077] Mass storage device 186, which may be implemented as a magneticor an optical disk drive, is preferably a non-volatile storage devicefor storing data and instructions for use by processor 182. The massstorage device 186 may store client/server information, code forcarrying out the methods of the invention, and computer instructions forthe processor. The computer instructions for implementing the methods ofthe present invention also may be stored in processor 182.

[0078] Portable storage medium drive 194 may operate in conjunction witha portable non-volatile storage medium, such as a floppy disk or othercomputer-readable medium, to input and output data and code to and fromthe computer system 180. According to an exemplary embodiment, themethod of the present invention is implemented using computerinstructions that are stored on such a portable medium and input to thecomputer system 180 via the portable storage medium drive 194.

[0079] The peripheral devices 188 may include any type of computersupport device, such as an input/output (I/O) interface, to addadditional functionality to the computer system 180. For example, theperipheral devices 188 may include a network interface card forinterfacing the computer system 180 to a network, a modem, and the like.

[0080] The input devices 192 provide a portion of a user interface andmay include an alphanumeric keypad or a pointing device such as a mouse,a trackball, a stylus, or cursor direction keys. Such devices provideadditional means for interfacing with a customized media list andcustomized media of the present invention.

[0081] The graphics subsystem 196 and the display 198 provide outputalternatives of the system 180. The display 198 may include a cathoderay tube (CRT) display, a liquid crystal display (LCD), or othersuitable devices that enable a user to view the customized media list orthe customized media of the invention. The graphics subsystem 196 mayreceive textual and graphical information and then process theinformation for output to the display 198.

[0082] The audio means 190 may include a sound card that receives audiosignals from a peripheral microphone. In addition, the audio means 190may include a processor for processing sound. The output devices 200 mayinclude suitable output devices such as speakers, printers, and thelike.

[0083] Each of the components of exemplary computer system 180 areintended to represent a broad category of computer components that arewell known in the art. Exemplary computer system 180 represents oneplatform that can be used for implementing the methods of the presentinvention. Numerous other platforms can also suffice, such asMacintosh-based platforms, platforms with different bus configurations,networked platforms, multi-processor platforms, other personalcomputers, workstations, mainframes, navigational systems, and the like.

[0084] Although the present invention has been described in terms of theexemplary embodiments, numerous modifications and/or additions to theabove-described embodiments would be readily apparent to one skilled inthe art. It is intended that the scope of the present invention extendsto all such modifications and/or additions and that the scope of thepresent invention is limited solely by the claims set forth below.

What is claimed is:
 1. A method of compensating for imaging offset of adot produced by an exposure unit, on a substrate, in an imaging system,the imaging system comprising an array of exposure units, with at leastone exposure unit misaligned in comparison with the other exposureunits, the method comprising: determining the imaging offset as adistance between the ideal dot position and the uncompensated dotposition for each exposure unit; and matching the uncompensated dotposition to the ideal dot position.
 2. The method of claim 1, whereinthe matching step comprises: delaying a formation of the dot on thesubstrate by an amount of time corresponding to the imaging offset. 3.The method of claim 1, wherein the matching step further comprises:determining a time factor based on the imaging offset.
 4. The method ofclaim 3, wherein the step of determining a time factor furthercomprises: determining a time factor that is proportional to a magnitudeof the distance of the imaging offset.
 5. The method of claim 1, whereinthe determining step further comprises: determining a magnitude of thedistance of the imaging offset.
 6. The method of claim 5, wherein thematching step further comprises: determining a time factor that isproportional to the magnitude of the distance of the imaging offset. 7.The method of claim 6, wherein the matching step further comprises:actuating the exposure unit at a time modified by the time factor. 8.The method of claim 5, wherein the determining step further comprises:determining a direction of the imaging offset.
 9. The method of claim 8,wherein the matching step further comprises: determining a time factorthat is proportional to the magnitude of the distance of the imagingoffset and that has a sign indicative of the direction of the imagingoffset.
 10. The method of claim 9, wherein the matching step furthercomprises: actuating the exposure unit at a time modified by the timefactor.
 11. A method of compensating for imaging offset of a dotproduced by an exposure unit, on a substrate, in an imaging system, theimaging system comprising an array of exposure units, with at least oneexposure unit misaligned, in comparison with the other exposure units,the method comprising: determining the imaging offset as a distancebetween the ideal dot position and the uncompensated dot position foreach exposure unit; and matching the uncompensated dot position to theideal dot position for each exposure unit.
 12. The method of claim 11,wherein the determining step further comprises: determining a directionof the imaging offset.
 13. The method of claim 12, wherein the matchingstep further comprises: determining a time factor for each exposure unitbased on the imaging offset.
 14. The method of claim 13, furthercomprising: actuating each exposure unit at a time modified by the timefactor thereof to form a latent image on the substrate.
 15. The methodof claim 12, further comprising: developing the latent image on sheetmaterial.
 16. The method of claim 11, wherein the determining stepfurther comprises: retrieving the imaging offset from a network.
 17. Themethod of claim 16, wherein the retrieving step further comprisesretrieving the imaging offset from the Internet.
 18. Anelectrophotographic (EPG) module for printing images free of imagingoffset, the EPG module comprising: a substrate; a light source includinga plurality of exposure units each for producing a dot on the substrate;the plurality of exposure units including at least one misalignedexposure unit that is out of alignment with the other exposure units;and each of the misaligned exposure units producing a dot at anuncompensated dot position when uncompensated for misaligned thereofsuch that each of the misaligned exposure units has an imaging offsetcorresponding to a distance defined between the uncompensated dotposition and an ideal dot position; and a matching device incommunication with the light source for causing each of the misalignedexposure units to be actuated based on the imaging offset to produce adot in the ideal dot position.
 19. The EPG module of claim 18, whereinthe matching device includes a device for causing the misalignedexposure units to be actuated at a time different from an idealactuation time that produces a dot in the ideal dot position.
 20. TheEPG module of claim 19, wherein the matching device includes a delayingdevice for causing the misaligned exposure units to be actuated at atime later than an ideal actuation time that produces a dot in the idealdot position.
 21. The EPG module of claim 19, wherein the matchingdevice includes a device for causing the misaligned exposure units to beactuated at a time earlier than an ideal actuation time that produces adot in the ideal dot position.
 22. The EPG module of claim 20, whereinthe matching device includes a device for causing the misalignedexposure units to be actuated at a time earlier than an ideal actuationtime that produces a dot in the ideal dot position.
 23. The EPG moduleof claim 18, further comprising a storage device for storing the imagingoffset for each of the exposure units, wherein the storage device storesidentification information unique to the EPG module.
 24. The EPG moduleof claim 18, wherein the substrate is a photoreceptor belt.
 25. Alight-emitting diode print head (LPH) comprising: a plurality ofexposure units each for producing a dot on the substrate; the pluralityof exposure units including at least one misaligned exposure unit thatis out of alignment with the other exposure units; and each of themisaligned exposure units producing a dot at an uncompensated dotposition when uncompensated for misalignment thereof such that each ofthe misaligned exposure units has an imaging offset corresponding to adistance defined between the uncompensated dot position and an ideal dotposition; and a matching device in communication with the storage deviceand the light source for causing each of the misaligned exposure unitsto be actuated based on the imaging offset to produce a dot in the idealdot position.
 26. The LPH of claim 25, wherein the matching devicecomprises a delaying device for causing the misaligned exposure units tobe actuated at a time different from an ideal actuation time thatproduces a dot in the ideal dot position.
 27. The LPH of claim 25,further comprising a storage device for storing the imaging offset foreach of the exposure units.
 28. The LPH of claim 27, wherein the storagedevice stores identification information unique to the EPG module. 29.An imaging system comprising: a photoreceptor belt; a light sourceincluding a plurality of exposure units each for producing a dot on thephotoreceptor belt; the plurality of exposure units including at leastone misaligned exposure unit that is out of alignment with the otherexposure units; and each of the misaligned exposure units producing adot at an uncompensated dot position when uncompensated for misalignmentthereof such that each of the misaligned exposure units has an imagingoffset corresponding to a distance defined between the uncompensated dotposition and an ideal dot position; and a processor in communicationwith the light source for causing each of the misaligned exposure unitsto be actuated based on the imaging offset to produce a dot in the idealdot position.
 30. The imaging system of claim 25, wherein image data isstored in an image buffer before being passed to a compensation module.31. The imaging system of claim 30, wherein the image data istransferred to the image buffer using direct memory access.
 32. Theimaging system of claim 30, wherein the image data is transferred fromthe image buffer using direct memory access.
 33. The imaging system ofclaim 30, wherein the image data is transferred to and from the imagebuffer using direct memory access.
 34. The imaging system of claim 30,wherein the processor delays the actuation of a misaligned exposure unitto be at a time later than an ideal actuation time that produces a dotin the ideal dot position.
 35. The imaging system of claim 30, whereinthe processor delays the actuation of a misaligned exposure unit to beat a time earlier than an ideal actuation time that produces a dot inthe ideal dot position.
 36. A computer-readable memory having storedthereon computer-readable instructions that, when loaded into a computersystem, cause the computer system to perform a method of compensatingfor imaging offset of dots produced by an array of exposure units on asubstrate in an imaging system, the dots having uncompensated dotpositions that are out of alignment with ideal dot positions, the methodcomprising: determining the imaging offset, for each exposure unit, as adistance between the ideal dot position and the uncompensated dotposition; and matching the uncompensated dot position to the ideal dotposition, for each exposure unit.
 37. A method of compensating forimaging offset of a dot produced by an exposure unit, on a substrate, inan imaging system, the imaging system comprising an array of exposureunits, with at least one exposure unit misaligned, in comparison withthe other exposure units, the method comprising: determining the imagingoffset as a distance between the ideal dot position and theuncompensated dot position for each exposure unit; determining adirection of the imaging offset; retrieving the imaging offset from anetwork; determining a time factor proportional to a magnitude of thedistance of the imaging offset; matching the uncompensated dot positionto the ideal dot position; actuating the exposure unit at a timemodified by the time factor; delaying a formation of the dot on thesubstrate by an amount of time corresponding to the imaging offset;storing the imaging offset for each of the exposure units in a storagedevice; and, developing the latent image on sheet material.